1. Field of the Invention
The present invention relates to information recording media which comprise a high density information recording means such as an optical information recording medium including an optical disk or a magnetic information recording medium including a fixed magnetic disk or a floppy disk, a tracking error signal detection apparatus for the information recording media, and an information recording apparatus which can precisely record, reproduce and erase information on the information recording media using the tracking error signal detection apparatus, and also relates to methods of adjusting an information recording apparatus.
2. Disclosure of the Prior Art
A track pitch of a conventional magnetic recording media on which information is recorded, such as a floppy disk. Therefore, the track pitch is much wider than that of an optical disk, which is about 1.6 .mu.m. Accordingly, a rough track location using a mechanical method such as a stepping motor has been sufficient. However, in order to realize a magnetic recording medium having a larger capacity, a track pitch from several .mu.m to several tens .mu.m is required. In this case, a precise track location becomes necessary.
FIG. 1 shows a configuration of a conventional magnetic recording apparatus which detects a tracking error signal by using light. In FIG. 1, a linearly polarized divergent beam 70 radiated from a semiconductor laser light source 10 is converted to a parallel beam by a collimator 20 and the parallel beam enters a polarizing beam splitter 30. All the parallel beam 70 entering the polarizing beam splitter 30 passes through the polarizing beam splitter 30 and enters a 1/4 wavelength plate 31. The parallel beam 70 is converted to a circularly polarized beam by passing through the 1/4 wavelength plate 31 and is focused on a magnetic recording medium 40 by an object lens 21.
FIG. 2 shows the relationship between the magnetic recording medium 40 and the focused light beam 70. The magnetic recording medium 40 has tracks Tn-1, Tn, Tn+1 . . . , which include the area on which information is recorded or reproduced by a magnetic head 99 with a certain pitch pt (approximately 20 .mu.m). Further, discrete guiding grooves Gn-1, Gn, Gn+1 . . . , which enable the optical detection of a signal synchronizing a tracking error signal and which enables rotations of the magnetic recording medium 40, are formed in the middle of adjacent tracks.
The beam 70 reflected and diffracted by the magnetic recording medium 40 passes through the object lens 21 again, and enters the 1/4 wavelength plate 31. By passing through the 1/4 wavelength plate 31 again, the beam 70 is converted to a linearly polarized beam having a 90.degree. phase change of the light source 10. All the beam passing through the 1/4 wavelength plate 31 is reflected by the polarizing beam splitter 30 and enters a photodetector 50. The incident light beam is converted into an electric signal by the photodetector 50 and inputted to a signal processing portion 80.
As illustrated in FIG. 1, the photodetector 50 has two light sensing portions 501, 502. Signals outputted from the light sensing portions 501, 502 are converted to voltage signals by current-voltage (I-V) converting portions 851, 852 respectively, and inputted to a differential operation part 871. The differential operation part 871 subtracts the two voltage signals outputted from the I-V converting portions 851, 852.
When a beam 70 from the optical system has a displacement x from the center of a guiding groove such as Gn on a magnetic recording medium 40, voltage signals v21, v22 outputted from the I-V converting portions 851, 852 become sine waves having opposite phases which can be approximately represented by the below mentioned formulae (1) and (2). The signals v2l, v22 can be illustrated as FIG. 3(a) and (b). EQU v21=-A.multidot.sin (2.pi.x/pt)+B (1) EQU v22=A.multidot.sin (2.pi.x/pt)+B (2)
In the formulae (1) and (2), A is an amplitude and B is a DC component.
A signal v23 outputted from the I-V converting portion 871 can be represented by the below mentioned formula (3) and outputted from a terminal 801 as the tracking error signal. EQU v23=2.multidot.A.multidot.sin (2.pi.x/pt) (3)
The signal v23 can be illustrated as FIG. 3(c). The tracking error signal v23 outputted from the terminal 801 is inputted to a driving portion 90 to adjust relative positions of a magnetic recording medium 40 and a base 95 including a tracking error signal detection optical system 100 and a magnetic head 99 for recording and reproducing information so as to form a desired track on the magnetic recording medium 40. The tracking error signal detection method is known as the push pull method.
(First Problem)
In a conventional magnetic recording apparatus using a magnetic head 99 for recording and reproducing information, and an optical system 100 for the detection of a tracking error signal, a distance d between a point S1 at which the magnetic head 99 contacts a magnetic recording medium 40 and a focal point S2 of a beam 70 from the optical system needs to be at least several hundred .mu.m to several mm. That is, the point S1 at which the magnetic head 99 contacts the magnetic recording medium 40 and the focal point S2 of the beam 70 scan different tracks on the magnetic recording medium 40.
In assembling a magnetic recording apparatus, the distance d is adjusted so as to have the working point of the tracking servo at the midpoint S3 of the signal amplitude of the tracking error signal v23 as illustrated in FIG. 3(c) when the point S1 is on a track of the magnetic recording medium 40. However, temperature or humidity change causes expansion or contraction of the magnetic recording medium 40 and the track pitch pt changes accordingly. Therefore, in the tracking operation at the point S3 using the tracking error signal v23 obtained from the optical system 100, the point S1 becomes off track and thereby drastically deteriorates the information reproduction characteristics.
In this case, for example, if a point S4 is the working point on the tracking error signal when the point S1 is on the track, a tracking servo can be enabled by applying an offset voltage to the tracking servo. However, the dynamic range of the orientation illustrated by the arrow D1 lowers and thereby deteriorate the followability in the case disturbance generates. Further, as the point S4 moves farther from the point S3, the servo gain of the tracking operation lowers. When the point S4 eventually reaches the point S5, a new problem occurs that the servo gain of the tracking becomes 0 thereby completely losing the tracking servo.
On the other hand, in an optical disk apparatus where the beam used to detect tracking error signals and the beam used to record information on the information recording medium are identical, a configuration forming a track on or between the guiding grooves so as to record and reproduce information with a further high density is proposed. However, in this configuration, when the relationship pt&gt;.lambda./NA is satisfied where .lambda. is a wavelength of the beam radiated from the light source, NA is an numerical aperture of the object lens at the information recording medium side, and pt is a cycle of marks or guiding grooves formed on the information recording medium to enable the detection of the tracking error signals, a problem similar to the above mentioned problem occurs when the predetermined angle between the beam focused by the object lens and the information recording medium can not be sustained. Specific examples include the case having a wavelength .lambda. of 650 nm, a numerical aperture NA of 0.6, a cycle pt of marks or guiding grooves of 1.48 .mu.m, and a substrate thickness for the information recording medium of 0.6 mm.
(Second Problem)
Dusts or flaws on the magnetic recording medium 40 change the reflection ratio of the magnetic recording medium 40 and the intensity of a light beam 70 reflected thereby accordingly. In this case, a problem occurs in that an offset occurs in the tracking error signal, and thus the magnetic head 99 can not be controlled on a desired track of the magnetic recording medium 99.
(Third Problem)
Moreover, as in the above mentioned prior art, if a stepping motor is used in the tracking driving portion 90 for a magnetic recording medium having a track pitch of several .mu.m to several tens .mu.m for seeking tracking error signals using a light beam, an off track generates which depends on the step width of the stepping motor. By making the step width narrower to reduce the off track amount, a problem occurs in that the time for detecting tracks becomes longer. These two problems can be solved by the use of a DC motor instead of a stepping motor in the tracking driving system. However, since mechanical positioning can not be controlled if a DC motor is employed in a tracking driving portion 90, a new problem occurs in that information can not be recorded or reproduced in a magnetic recording medium having a track pitch of 188 .mu.m, which is now widely used.
(Fourth Problem)
Further, an optically optimum value for a numerical aperture NA of the object lens 21 for a magnetic recording medium having a track pitch of 50 .mu.m is about 0.017. However, when an angular dislocation .theta. exists between the beam 70 focused by the object lens 21 and the magnetic recording medium 40, the beam 70 reflected by the magnetic recording medium 40 can not enter the aperture. Therefore a problem occurs in that the quantity of the light beam introduced to the photodetector 50 decreases and thus the tracking operation becomes unstable. The relationship between a performance function Ev with respect to the angular dislocation .theta. (Ev=0.5.multidot.tan(2.multidot..theta.)/NA) and a quantity of light I of the beam 70 introduced by the photodetector 50 is shown in FIG. 4. With a numerical aperture NA for the object lens 21 of 0.017, the angular dislocation .theta. is 0.97 when the quantity of light I of the beam 70 on the photodetector 50 is 0, namely, the performance function Ev is 1. In this case, the tracking error signal can not be obtained at all.